Mirror blanks may be used for various apparatus including telescopes. Typically a mirror blank includes at least one optical surface. The optical surface may be planar, spherically or aspherically shaped, dependent upon its desired use. The optical surface is typically polished and coated with a reflective coating to provide a mirror.
As mirror blanks become larger, such as those used for space telescopes where their diameters can be greater than 1 m, their design and manufacture become more complex. Mirror blanks may be formed using multiple polygonal shaped glass plates configured to maintain the overall mirror shape. Another design involves the use of a monolithic glass plate with a supporting core structure. In all designs, it is desirable to provide a lightweight mirror blank, so that the supporting structure is designed to be a lightweight core sealed against the glass plate.
For either design, the quality of seals between the core and the plate are typically tested to determine if they are adequately sealed. A flaw in a seal may be defined as an air gap between the core structure and the plate. The quality of the seals in these mirror blanks is typically confirmed by visual inspection and by destructively testing a portion of the mirror blank to find unacceptable flaws.
While many flaws may be detected by visible inspection alone, smaller flaws may go undetected. The visual inspection process may produce a qualitative analysis. However, the inspection is subjective in that a significant flaw determined by one inspector may be determined to be a minor flaw by another inspector. Also, the detection of the same flaw by visual inspection alone may not be repeatable. As the mirror blanks become larger or subject to applications requiring more stringent quality control, the flaw size, depth and concentration of multiple flaws may become more important. Thus, there is a need to develop a quantitative measure to measure mirror blank seal quality.